1. Field of the Invention
The present invention relates to a magnetic recording medium such as a magnetic tape, a magnetic disc, etc. More specifically, this invention relates to a magnetic recording medium provided with both a sufficient mechanical strength and a travel stability, even when its total thickness is adjusted to 12.5 .mu.m or less.
2. Description of the Prior Art
Recently, a magnetic recording medium which can keep recording signals for many hours has been required. As a result, there exists a necessity to reduce the total thickness of the recording medium as Thin as possible, in particular in the magnetic tape (e.g., a video tape) accommodated in a casing under wound-up condition. Here, however, when the thickness of the tape is simply thinned, since the mechanical strength thereof is reduced, there arise various problems in that the medium itself is subjected to damage (e.g., breakage, wrinkles, etc.) To solve these problems, such a magnetic recording medium has been proposed that a layer (using flat inorganic powder and carbon black as a filler) is sandwiched either between a rear side of a non-magnetic supporter and a magnetic layer or between the supporter and a back coat layer or layers with such filler are sandwiched between the supporter and the magnetic layer and between the supporter and the back coat layer. Such a magnetic recording medium is disclosed in Japanese Published Unexamined (Kokai) Patent Application Nos. 64-60819, 4-285725, 4-285726, and 4-195819, for instance.
In The above-mentioned prior art magnetic recording medium, however, when the total thickness thereof is reduced down to as thin as 12.5 .mu.m or less (a target value of the Inventors), it is impossible to obtain a sufficient mechanical strength and a sufficient travel stability. In practice, the Inventors manufactured the above-mentioned prior art recording medium whose thickness was reduced down to 12.5 .mu.m or less in total thickness for various tests. The test results were such that the tapes were damaged, broken or wrinkled due to the contact with a guide pole for guiding the tape travel.
In addition, in the above-mentioned Patent 64-60819, the quantity of carbon black contained in the back coat layer as an additive is defined as 50 to 300 parts by weight (referred to as wt. parts, sometimes hereinafter). However, when this tape is used as a magnetic tape with a thickness equal to or less than 12.5 .mu.m, since the added carbon black is excessive, the mechanical strength of the magnetic tape is extremely reduced, as a result the magnetic tape is subjected to edge damage (e.g., breakage, wrinkles, etc.)
As described above, when the thickness of the prior art magnetic recording tape is simply reduced as thin as 12.5 .mu.m or less, the tape cannot be used in practice.